Hot and cold rolled steels are both used to fabricate a wide variety of goods, but each have unique properties depending on the temperature at which the steel has been worked or rolled in relation to the steel's critical temperature range, i.e., a temperature range through which the properties of the steel change dramatically. Hot rolled steel provided by steel mills is coated with a tough skin of steel oxide primarily composed of Fe3O4, known as “mill scale,” whereas cold rolled steel has a smooth surface with very few blemishes. Hot rolled steel also has more rust that is tightly bound to the surface than does cold rolled steel. On the other hand cold rolled steel is normally “oiled” prior to shipment and said oils are desired to be removed prior to the welding, laser cutting and coatings processes.
The manufacturing/fabrication industry over the years has utilized both grit blast cleaning and pickling and oiling solutions to aid in the preparation of hot rolled steel for optimizing coatings adhesion. Blast cleaning has been found to be a flexible method of removing up to about 95 percent of the mill scale, it is nevertheless a high cost and maintenance-intensive surface preparation method. Not only does the abrasive itself have a limited life and high replacement cost, but grit blast cleaning also presents major safety concerns due to eye injuries and poses many serious operational problems. Such problems include rapid wear of mechanical moving parts due to fouling and contamination of lubricants, as well as part breakage due to abrasion and jamming. For example, there have been instances of shutdowns due to augers snapping upon being jammed with abrasive. Also, particle elevators have become jammed. Moreover, the presence of moisture is a problem with grit blast cleaning methods because the moisture causes the abrasive to agglomerate and form solid cakes, which are difficult to dislodge from the equipment, and can jam moving parts while compounding the problem of abrasive loss. Once the grit becomes caked, it then needs to be removed and replaced with new grit. There can be approximately forty to fifty-five 55-gallon drums to replace by hand, requiring about eight hours and three workers, and a loss of an entire production shift, in addition to the cost of the grit. Finally, the quality of finish attainable by grit blast cleaning is limited as a practical matter. In most commercial applications in which much of the mill scale is removed from metal surfaces, a significant amount of rust remains on hard-to-reach areas. Moreover, areas that are easily accessible are left with a 2-2.5 mil surface profile, which is an uneven surface requiring almost twice the amount of paint to coat. Operator inconsistencies regarding application of blasted grit can negatively impact both the end “profile” of the steel and thru heat, affect the steel's structural ability. Consequently, ever increasing input costs coupled with the manufacturers/fabricators desire to achieve optimum results paved the path for noted invention.
The Pickling and Oiling process demands large acquisition costs and input costs. Although the end result removes surface rust and mill scale—the end product the manufacturer/fabricator is left with is “oiled” steel to inhibit rusting of the surface for storage and transportation. Removal of the oils applied to the steel is required prior to the welding (smoke, splatter), laser cutting (piercing ability, diffusion and tip wear) and coatings (adhesion, corrosion assistants, fish eyes). This method resolves certain issues for the supplier of the steel, but still requires further processing by the manufacturer/fabricator—increasing overhead, input and labour costs. Also, certain coatings require a neutral ph on the substrate and pickling solutions have been noted to increase the acidity of the steel.
More particularly, there are a number of methods for cleaning iron and steel surfaces. Known methods include acid pickling, acid cleaning, alkaline descaling, salt bath descaling, brushing, and abrasive blasting or tumbling.
Other methods for surface cleaning and preparation include power tool cleaning, water washing, and abrasive injection in water. Power tool cleaning can remove a very high degree of rust and mill scale, but like blast cleaning, power tool cleaning generates large quantities of dust, while consuming large amounts of energy and requiring frequent maintenance and replacement of worn parts. Water washing equipment requires specialized components for operating at medium to high pressure (3000 psig or higher). For example, a pressure pump, a specialized lance and a nozzle assembly are required, in addition to large volumes of water. This technique can remove loose rust but will not effectively remove tight rust or mill scale. Abrasive injection in water may provide a greater ability to remove rust and mill scale. However, abrasive injection in water raises most of the concerns of ordinary blast cleaning, in addition to consuming large volumes of water.
Due to the drawbacks of these alternative methods, mill scale and rust on large articles made of hot rolled steel has conventionally been removed by blast cleaning, a method of removing mill scale, rust, rust scale, paint or foreign matter by the use of abrasives propelled through nozzles or by centrifugal wheels. Typically, a “Commercial Blast Cleaned Surface Finish” is sought, which is defined by the Steel Structures Painting Council (SSPC) as one from which all oil, grease, dirt, rust scale and foreign matter have been completely removed from the surface, and all rust, mill scale and old paint have been completely removed except for slight shadows, streaks, or discolorations caused by rust stain, mill scale oxides or slight, tight residues of paint or coating that may remain. If the surface is pitted, slight residues of rust or paint may be found in the bottom of pits. Under this definition, at least two-thirds of each square inch of surface area is free of all visible residues and the remainder is limited to the above-mentioned light discoloration, slight staining or tight residues.